Activity of vascular smooth muscle of large pulmonary conductance and capacitance vessels will be studied in closed chest dogs using a catheter-tip pressure-diameter gauge. Slow sinusoidal oscillation in pulmonary arterial volume will be induced by pumping in order to obtain pressure-diameter relationship curves over a relatively large range of pressure. These PDR curves will be analyzed for the vascular segment studied in terms of static and dynamic compliance. Total pulmonary compliance will be obtained simultaneously using the pressure-slope method. The effect of smooth muscle activity on PDR curves and these other variables will be defined quantitatively. The effects of physiological interventions, such as changes in posture, blood volume, venous return, etc. on smooth muscle activity, will be investigated. Also, the effects of vasoactive agents will be studied, dose response curves obtained where possible and the involvement of specific receptors elucidated.